In recent years digital still cameras (hereinafter referred to simply as digital cameras) that enable input of picture image information, such as photographed landscapes and portraits, into a personal computer are rapidly becoming more popular. Additionally, portable telephones that include portable cameras that incorporate compact imaging modules with high functionality are rapidly becoming more popular. Furthermore, including an imaging module in compact information terminal equipment, such as PDAs (Personal Digital Assistants), is becoming popular.
In such devices that include an imaging function, an image pickup element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), is used to provide the imaging function. Recently, advancements in the miniaturization of such image pickup elements have been rapidly increasing. This has resulted in a desire for the main body of such devices and the imaging lens system used in the imaging module to also be further miniaturized. Additionally, image pickup elements with a larger number of pixels in the same area have been developed in order to achieve higher image quality, which creates a demand for higher resolution lens systems that are still very compact, as well as higher contrast performance. Japanese Laid-Open Patent Application 2000-258684 describes exemplary single focus imaging lenses for such devices that include only two lens elements.
As stated above, recent image pickup elements are smaller and provide more pixels in a given detector area, which helps meet demands of higher resolution and more compactness that are especially required in imaging lenses for digital cameras. On the other hand, considerations of small cost and compactness have been the main considerations for imaging lenses for compact information terminal equipment, such as portable telephones with cameras. However, more recently, such devices have incorporated megapixel detectors (detectors that detect one million or more pixels), indicating increasing demand for higher performance in these devices as well, which has been accompanied by demands to make such devices smaller and to improve other performance properties. Therefore, development of lens systems with a wide range of applications based on properly balancing considerations of cost, performance, and compactness is desired.
For example, as an imaging lens for compact information terminal equipment having a large number of pixels, there has been developed a lens system having three lens components, each of which may be a lens element, with at least two lens elements being made of plastic, while the third lens element may be made of plastic or glass. However, in order to meet recent demands for greater miniaturization, a lens that uses a smaller number of lens components and lens elements, but which is equivalent in performance to these conventional lenses, is desired.
Although the lenses described in Japanese Laid-Open Patent Application 2000-258684, referenced above, each have a two-component, two-element lens construction, which includes aspheric surfaces, a lens system that is even more compact and higher in performance is desired. Particularly, when a small-size image pickup element is used, a lens system that well corrects lateral color is desired as the lens system because lateral color readily becomes noticeable.
Additionally, it is desirable to well correct distortion in order to achieve higher performance. In order to do this, a common type of imaging lens is frequently designed by considering optical distortion, but it is considered that an improvement in the perceived image of a photographed picture on a monitor screen can be made by considering the TV distortion related directly to the monitor in the design of the imaging lens. For example, the apparent distortion of the entire image can be made unnoticeable by balancing the TV distortion with the optical distortion.
Concepts of optical distortion and TV distortion will now be described with reference to FIG. 7. FIG. 7 schematically shows a rectangular object that is imaged via an optical system and displayed on a TV screen. In FIG. 7, a broken line 10 shows the rectangular shape that would be the ideal representation of the rectangular object, and a solid line 11 shows the shape of an actual displayed image. With reference to FIG. 7, if the ideal image height is y0 and the actual image height is y, the amount of aberration of optical distortion D is generally expressed by the following equation:D=[(y−y0)/y0]×100 (%).Namely, the optical distortion D is defined by dividing the difference between the actual image height y and the ideal image height y0 by the ideal image height y0 and multiplying the quotient obtained by 100 percent in order to express the optical distortion in percentage terms.
On the other hand, again with reference to FIG. 7, the TV distortion Dt is defined by dividing the depth of curvature Δh of the long side of the actual image that ideally would have no curvature by twice the vertical height h (i.e., as measured from the optical axis, which corresponds to the center of the TV image) of a shorter side of the actual image and multiplying the quotient obtained by 100 percent in order to express the optical distortion in percentage terms. Thus, the TV distortion is defined by the following equation:Dt=(Δh/2h)×100(%).